2.8 - Is it possible to assign (override) a material to a whole collection?
You can override the materials from linked sources that are part of the same collection, just the same way you can override not linked ones.
In the scene context section you can find the options for override, you can select a single material and that will be used for all objects.

From the blender manual:

Material Override
Overrides all material settings to use the Material chosen here.
Examples of where this might be used:
To check lighting by using a plain diffuse material on all objects.
Render a wireframe of the scene.
Create a custom render pass such as an anti-aliased matte or global coordinates.